(a) Field of the Invention
The present invention relates to a solid-state image sensor and, more particularly, to an interline charge-coupled device (CCD) image sensor of a simultaneous reading type having transfer electrodes formed by three-layer conductor. The present invention also relates to a method for fabrication thereof.
(b) Description of the Related Art
Recently, video cameras comprising solid-state image sensor are widely used for business or home use. These conventional video cameras have generally adopted an interlace scanning system which scans every other horizontal signal line during a single frame corresponding to a television system (for instance, NTSC system and PAL system). On the other hand, image pick-up cameras for personal computers, developed actively in recent years, use a simultaneous reading scheme which simultaneously reads all the pixels of the image sensor, from the viewpoint of obtaining a high resolution still picture and easiness of output to the displays. This system is called simultaneous reading system, sequential scanning system, non-interlacing scanning system or progressive scanning system. The solid-state image sensor of the simultaneous reading type reads out signal charge from all of the pixels simultaneously and independently. A typical solid-state image sensor of the simultaneous reading type is more specifically described in the followings.
FIG. 1 is a top plan view of an active area of a conventional interline CCD image sensor of the simultaneous reading type. The active area of the image sensor comprises a imaging region 11, a horizontal CCD 12, and an output section (or charge detecting section) 13. Imaging region 11 comprises a plurality of photodiodes 14 arranged in a two-dimensional matrix for accumulating therein signal charge obtained by photoelectric conversion, and a vertical CCD 15 disposed between adjacent columns of photodiodes 14 for transferring the signal charge in the vertical direction. A transfer gate area 16 is also disposed between a column of photodiodes 14 and a corresponding vertical CCD 15 for reading the signal charge from the column of photodiodes 14 to the vertical CCD 15. The rest of the imaging region 11 is formed as an element isolation region 17.
In operation, the signal charge, obtained by photoelectric conversion in each photodiode 14 and accumulated therein within a certain period of time, is read out to vertical CCD 15 through transfer gate area 16. The signal charge read out from each horizontal line of photodiodes toward respective vertical CCDs 15 is transferred therein toward a horizontal CCD 12 in the active area step by step using a driving signal. The signal charge transferred to horizontal CCD 12 is then transferred in the horizontal direction toward output section 13 for detection of a two-dimensional image.
FIG. 2 is a top plane view of one of the pixels in a typical image sensor of the simultaneous reading type, and FIG. 2A is a cross-sectional view of the pixel taken along line Axe2x80x94A in FIG. 2. The pixel comprises photodiode 14, vertical CCD 15, transfer gate area 16 and element isolation region 17. Vertical CCD 15 comprises a channel and four associated vertical transfer electrodes 18 to 21. At least one of the four vertical transfer electrodes 18 to 21, for example, vertical transfer electrode 22 has an additional function as a reading electrode for reading the signal charge from photodiode 14 to vertical CCD 15.
Vertical transfer electrodes 18 to 21 are formed by three layers of polysilicon. The three-layer polysilicon films are consecutively referred to as a first layer polysilicon film, a second layer polysilicon film, and a third layer polysilicon film from the bottom to the top in this text. Vertical transfer electrode 18 formed by the first layer polysilicon film and shown by a dotted line extends in the horizontal direction as viewed in the drawing. Namely, vertical transfer electrode 18 extends in the imaging region across element isolation region 17 which separates adjacent photodiodes in the vertical direction. Vertical transfer electrodes 19 and 20 formed by the second layer polysilicon film extend in the horizontal direction in the imaging region, overlying the side-wall of vertical transfer electrode 18. Vertical transfer electrode 21 formed by the third layer polysilicon film overlies a portion of the vertical CCD channel not covered by vertical transfer electrodes 18, 19 and 20, and extends in the horizontal direction in the imaging region, overlying vertical transfer electrode 18.
As shown in FIG. 2A, a P-well 24 is formed on the main surface of an N-type silicon substrate 23, and an N-type buried layer 25 is formed thereon. Vertical transfer electrode 18 is formed thereon with an intervention of a first isolation film 26, vertical transfer electrodes 19 and 20 are formed thereon with an intervention of a second insulation film 26, and vertical transfer electrode 21 is formed thereon with an intervention of a third isolation film 26. These vertical electrodes formed by three layer polysilicon partly overlap one another.
FIG. 3 is a longitudinal-sectional view of horizontal CCD 12 taken along the charge transfer direction. In horizontal CCD 12, P-well 24 is formed on the main surface of N-type silicon substrate 23, and N-type buried layer 25 constituting a transfer channel and including Nxe2x88x92regions 27 is formed thereon. A horizontal accumulation electrode 28 made of the first layer polysilicon film and a horizontal barrier electrode 29 made of the second layer polysilicon film are consecutively overlaid thereon with an intervention of insulation films 26. Horizontal accumulation electrode 28a and a corresponding horizontal barrier electrode 29a are electrically connected together, and a horizontal accumulation electrode 28b and a corresponding horizontal barrier electrode 29b are electrically connected together. These electrodes of horizontal CCD 12 are driven by a two-phase driving signal including a pair of horizontal transfer pulse trains xcfx86H1 and xcfx86H2. Nxe2x88x92-type buried layer 27 underlying horizontal barrier electrodes 29a and 29b have a lower impurity concentration and a higher electric potential for the signal charge than N-type buried layer 25 underlying horizontal accumulation electrodes 28a and 28b. 
FIG. 4 is a top plan view of a boundary between the imaging region 11 and horizontal CCD 15, showing conventional technique of a connection between the vertical CCD and the horizontal CCD. The structure shown therein is disclosed in, for instance, JP-B-4(1992)-19752. The following description is made based on a exemplified structure wherein a final electrode 30 of vertical CCD is implemented by the first layer polysilicon film or first vertical electrode. Horizontal barrier electrode 29a is formed by the second layer polysilicon film to partly overlap final vertical transfer electrode 30. Although horizontal barrier electrode 29a extends toward vertical CCD 15 to overlie the side-wall of final vertical transfer electrode 30 formed on the channel of vertical CCD 15, adjacent horizontal barrier electrodes 29a corresponding to adjacent vertical CCDs 15 are separated from each other in element isolation region 17. At side-wall 31 of horizontal accumulation electrode 28a, as shown in FIG. 4, adjacent horizontal barrier electrodes 29a and 29b receiving different pulses are disposed with a distance of about 1 micrometer therebetween.
FIG. 5 is a perspective view of the vicinity of horizontal accumulation electrode 28a, as viewed in the direction of arrow xe2x80x9cPxe2x80x9d in FIG. 4 for showing the side-wall 31 of horizontal accumulation electrode 28a. The structure of FIG. 4 is obtained as follows.
A first layer polysilicon film is deposited and patterned for obtaining horizontal accumulation electrodes 28, followed by formation of insulation film 26 such as thermal oxide film or CVD oxide film. Subsequently, a second layer polysilicon film is deposited on the entire surface, followed by patterning thereof using a resist mask to obtain horizontal barrier electrodes 29a and 29b. During the patterning of the second layer polysilicon film, if etching residues of the second layer polysilicon film remain on side-wall 31, a short-circuit failure may occur between horizontal barrier electrodes 29a and 29b receiving different driving pulses, which results in a problem of lower fabrication yield of the image sensor.
For removing the etching residues from side-wall 31, the amount of side etching may be increased. However, increase in the amount of side etching increases the variations of the product dimensions, which may cause variations in the transfer characteristics of the CCD, especially in the case of a solid-state image sensor having pixel dimensions of 5xc3x975 micrometers or smaller. In this description of the conventional technique, horizontal accumulation electrode 28a and horizontal barrier electrode 29a and 29b are formed by the first and the second layer polysilicon films, respectively. Similar problems will be involved when these electrodes are formed by the second and the third layer polysilicon films.
Another method, such as an additional etching using a resist mask having a small opening for exposing side-wall 31, may be used for removing the etching residues from side-wall 31. However, since side-wall 31 is disposed in the active area, wherein the insulation films having a thickness as low as several dozens of micrometers are formed, device characteristics may be degraded by the etching damage. In addition, it is not preferable to increase the number of photolithographic steps. Even if the additional etching on the element isolation region does not degrades the device characteristics, it is not easy to form a resist mark having a small opening as low as 1 micrometer wide or less without an offset.
It is therefore an object of the present invention to provide a solid-state image sensor having improved device characteristics and capable of being fabricated with an improved fabrication yield. It is another object of the present invention to provide a method for fabricating the solid-state image sensor.
The present invention provides, in a first aspect thereof, a charge-coupled device (CCD) image sensor comprising an active area and a field area, the active area receiving therein an array of photodiodes arranged in a two-dimensional matrix, a vertical CCD for transferring therein signal charge transferred from each column of the photodiodes, a horizontal CCD for transferring the signal charge transferred by the vertical CCD, and an output section for detecting the signal charge transferred by the horizontal CCD to output a two-dimensional image, the vertical CCD having a transfer channel and a plurality of associated vertical transfer electrodes for receiving a driving signal including a plurality of pulse trains, the field area receiving therein a plurality of bus lines corresponding to the vertical transfer electrodes, the vertical transfer electrodes being implemented by a plurality of conductive films including a first layer film and a second layer film, the second layer film covering substantially an entire side wall of the first layer film in the active area, the first layer film protruding from the second and third layer films in the field area.
The present invention also provides, in a second aspect thereof, a method for fabricating the CCD image sensor as described above. The method comprises the steps of etching said second layer film using a first resist mask and additionally etching residues of said second layer film by using a second resist mask having an opening exposing a portion of said first layer film not covered by said second layer film in said field area.
In accordance with the CCD image sensor according to the present invention, a short circuit failure occurring between the electrodes formed by the second layer film along the surface of side-wall of the first layer film can be prevented by preventing the etching residues of the second layer film remaining on the first layer film or providing a large distance between the electrodes formed by the second layer film along the surface of the side-wall of the first layer film.
The above and other objects, features and advantages of the present invention will be more apparent from the following description, referring to the accompanying drawings.